Process of producing an oxide coating on magnesium-aluminum alloys



United States Patent 3,247,026 PRUCESS 0F PRODUCING AN OXIDE COATING (3N MAGNESIUM-ALUMINUM ALLOYS Marshall H. Switzer, Glen Ellyn, Ill., assignor to Continental Can Company, Inc., New York, N.Y., a corporation of New York No Drawing. Filed Feb. 20, 1962, Ser. No. 174,385

7 Claims. (Cl. 1486.3)

The present invention generally deals with the formation of stable films on the surface of magnesium-aluminum alloy articles so as to render such articles permanently receptive to organic enamels. The invention more specifically deals with the method and the film thus formed by the action of saturated steam or boiling water upon the surface of a pre-cleaned magnesium-aluminum alloy article so that the surface of the metal article will be rendered receptively stable to the application of an organic enamel so that the enamel will remain permanently secured thereto.

During the past decade, use of sheet material of aluminum and its alloys has been gaining widespread acceptance as an alternative to the use of chemically treated electrolytic tinplate in the manufacture of cans. In many cases, the use of aluminum sheet material is preferred by the canning industry for special can applications, primarily due to the high strength/weight ratio of the material. In the vast majority of cases, the end use to which such cans are put dictates the utilization of a coating of an organic enamel upon at least the inner surfaces of the metal cans. It is also apparent that, when such or ganic coatings are utilized, it is an absolute necessity that they become permanently adherent to the metal substrates. In most applications, it is found desirable to coat the metal surface with the organic enamel prior to the formation of the can.

Magnesium containing alloys of aluminum have been found particularly desirable in the manufacture of cans for the food industry, e.g., orange juice containers, due primarily to their high strength and temper characteristics. The use of these magnesium-aluminum alloys has hithertofore, however, been seriously restricted because of their generally known and accepted characteristic of exhibiting extremely poor adhesion for organic enamels. In the past it has therefore been necessary to develop highly specialized and thus expensive organic enamels for use with the magnesium-aluminum alloys and even then such tailor-made enamels have not met with a high degree of commercial acceptability. Because of this serious shortcoming of magnesium-aluminum alloys for the can industry, the use of this otherwise highly acceptable magnesium-aluminum alloy group has been drastically limited.

It is, therefore, a principal object of the present invention to permanently render the surfaces of such magnesium-aluminum alloys permanently adherent to organic enamels.

It is also an object of the invention to provide a method of increasing the adherence of organic enamels to the surface of magnesium-aluminum alloys.

Another object of the invention is to provide a method of treating an article of magnesium-aluminum alloy so that the surface of the metal will be permanently adherent to organic enamels.

Still another object of the invention is to provide an article formed of a magnesium-aluminum alloy and having a permanently adherent coating of an organic enamel.

A further object of the invention is the provision of a simple and inexpensive method of treating and stabilizing the surface of a magnesium-aluminum alloy article so that it is permanently adherent to organic enamels.

A still further object of the invention is the process of preparing such alloy surfaces by mechanically or chemically cleaning the surface of a magnesium-aluminum alloy article to remove therefrom the normally present oxygen-containing metal compounds which reduce the permanent adherence of organic enamels to the surface of the metal, and subjecting the thus cleaned surface to the action of steam or boiling water for an extremely short period of time.

A further object of the invention is a process for providing the surface of a magnesium-aluminum alloy article with a tightly adherent thin flexible magnesium-aluminum polymeric oxide film.

A further object of the invention is a process of quickly and simply providing the surface of a magnesium-aluminum alloy article With a magnesium-aluminum polymeric oxide film which film affords organic enamels permanent adherence to the surface of the metal article.

A still further object of the invention is a method of forming a highly resistant and permanent metal polymeric oxide film on the surface of an aluminum alloy article in a shorter period of time than hitherto has been possible.

With the above and other objects in view, as will appear hereinafter, the nature of the invention will be more clearly understood by reference to the following detailed description.

The use of a boehmite film formed on an aluminum metal surface as a substrate upon which organic enamels are then coated is known in the art. These films are believed to be composed of polymeric hydrated aluminum oxides of uncertain molecular weight. Enhanced organic enamel adhesion is effected when such enamel is employed for coating aluminum surfaces having the stated boehmite film thereon. Boehmite films also exhibit various extents of hydration, molecular weight, density, and ability as substrates in permanently retaining organic enamels. Their representative structures are generally accepted to include the following:

(A) on Al-OH (B) 03 /OH /AlO-Al\ on on (c) on )11 on AlOAl-O--Al on on o or: OH

where Formula D is representative of the basic polymer unit of S-boehmite. Films of S-boehmite, as a substrate on the surface of aluminum sheet, have been found to be most satisfactory in increasing the extent to which organic enamels adhere to the surface of the aluminum metal sheet. Substrate films of S-boehmite have been formed on the surface of aluminum sheet articles by the subjection of the sheet to the action of boiling water or steam for a period of one to 20 hours. Other forms of boehmite film on aluminum articles, which exhibit less well defined crystal patterns, lower densities and a lesser extent of hydration have been formed by the action of boiling water or steam on the surface of aluminum sheets for shorter periods of time than required for the more desirable S-boehmite form. The boehmite films other than the S-boehmite type exhibit a far less tendency to increase the extent of adhesion of an organic enamel to the surface of the treated aluminum. These other types of boehmite films, although not taking the very extensive treatment time required for the S-boehmite, still require an uneconomically long treatment time. The shortest treatment time reported has been 2 to 15 minutes by Altenthol in German Patent 965,715.

The problem presented for solution is not merely that of organic enamelihg upon aluminum, but more precisely to such enameling upon alloys of magnesium and aluminum. Organic enameled aluminum sheets, although not exhibiting the almost perfect adherence associated with electrolytic tinplate, have not presented the grave problems of organic enamel separation from the base metal as compared with organic enamel separation from articles of magnesium-aluminum alloy.

It has now been found that by subjecting the precleaned surface of a magnesium-aluminum alloy article to the action of steam for a time period as little as 2 to 5 seconds, polymeric films are produced on the surface of the magnesium-aluminum alloy and organic enamels permanently adhere thereto. That is surprising in view of the past knowledge that boehmite films, upon aluminum, exhibiting like characteristics concerning organic enamel retention, required one to 20 hours treatment under the action of boiling water or steam. This unexpected result appears to flow from the magnesium present in the magnesiumaluminum alloy and the exhibition of a high synergistic action in the over-all process. It is further believed that the film exhibited at the surface of a magnesium-aluminum alloy article is composed of a polymeric hydrated oxide of aluminum similar to the aforementioned boehmite films but one in which the polymeric structure includes both aluminum and magnesium.

Extensive tests have been carried out on various selected magnesium-aluminum alloys along with a standard aluminum alloy containing no magnesium, so that the results of the tests would be truly comparative. The materials used in testing were as follows: #3003 (aluminum with no magnesium), #5052 (2.5% magnesium-aluminum alloy), #5154 (3.5% magnesium-aluminum alloy) and #5086 (4.0% magnesium-aluminum alloy). The stated percentages are those of magnesium content. One-half of the sample strips were subjected to mechanical abrasion by a series of rotating wire brushes so as to clear any metal oxides or other compounds, which normally interfere with the adhesion of an organic enamel, from the surface of the alloys: the other half were not so abraded. The test strips were also divided into further groups comprising those strips which received a steam treatment and those that did not. A further division was then made so that half of the test strips were prebaked before enamel coating and half were immediately coated with representative organic enamels. The steam treatment was by saturated steam at degrees C. for a period of 5 seconds while those that received no abrasion received a 10 second steam treatment. The prebaked test samples were heated in a commercial oven for 12 minutes at approximately 200 degrees C. The enamel applicator was located so as to immediately coat each test sample as it emerged from the prebake oven or after it had been dried following a steam treatment.

Over 1,000 samples were evaluated under otherwise identical conditions so as to fairly determine the extent of the enamel adhesion to the surfaces of the variously treated test strips. A numerical score was assigned to each evaluated test sample on the basis of 1,000 times the average fraction of enamel removed from the test area, that is, a score of 1,000 represents 100 percent enamel adhesion failure while a score of 0 indicates that no enamel was removed from the test area of the thus treated and tested samples. The results of these tests are set forth in Tables I and II:

TABLE I.-ORGANIC ENAMEL ADHESION OF NONABRADED ALLOY SURFACES Enamel Alloy Steam Prebake 127 224 80GLD 139C 126ALN 70C 70 llQD A151 No 10 22 0 10 2 12 11 2 N o No 6 22 0 8 8 8 8 1 S N o 0 11 0 163 0 174 43 953 No No 1 24 0 2 191 30 062 S N N o O 25 0 236 l 331 212 1000 S N 0 15 280 0 97 7 68 108 1000 B 46 92 0 86 5 202 194 1000 N N o 13 65 0 400 0 235 218 1000 S N o 6 121 0 188 1 238 107 1000 B 61 226 21 144 6 581 092 1000 TABLE II.ORGANIO ENAMEL ADHESION OF ABRADED ALLOY SURFACES Further tests were made utilizing, instead of abrasion as the cleaning medium for the surface of the metal sheet, an alkali detergent bath followed by an acid bath. T he test results obtained by substituting the chemically cleaned test samples were directly comparable to the test results with the abraded samples and have not been included in the above tabulations; that is, the test results as shown in Table I also indicate the properties of those cleaned by chemical means. For practical considerations, the preferred commercial embodiment of this invention includes the chemical cleaning of the metal surface prior to the steam application.

The duration of steam application to the cleaned test sheets may vary between 2 and 30 seconds. For economy, it is desirable to make use of the shortest steam treatment time possible, hence, work was also done at treatment times of less than 2 seconds but the results were not consistent. The application of steam to the cleaned surface of magnesium-aluminum alloy sheet for times from 2 to 30 seconds was found to produce the highly improved organic enamel adhesion sought. It is unnecessary to subject the surface of the metal to steam treatment for more than 30 seconds though such is not harmful and significant results are present after 2 seconds of actual contact exposure. The times of 5 and seconds utilized above for the treatment of the abrasion or chemically cleaned and uncleaned test samples, respectively, were selected primarily from commercial considerations, such times being convenient in large scale plant operations.

The results set out in Tables I and 11 indicate the improved results with samples which were cleaned prior to further treatment. This indicates that the surfaces of the uncleaned magnesium-aluminum alloys and to a slight extent the uncleaned non-magnesium control, normally have interfering compounds present. The higher degree of organic enamel adherence obtained with the cleaned samples is notable, Where such samples have been subjected alternatively to a steam or a prebake operation. However, when the samples which had been cleaned but not steam treated, were subjected to a prebake, a marked difference of organic enamel adhesion occurred. While this suggests that magnesium-aluminum alloys, which have been immediately enamel coated after cleaning, will Enamel Alloy Steam Prebakc 127 224 SOGLD 139C 12GALN 70C 70 11QD A151 No 0 0 0 2 0 1 4 7 No S N o 0 0 0 0 0 9 2 1 B 0 1 0 1 0 4 1 0 N0 0 8 0 0 1 O 0 0 N 0 S No 0 0 0 O 0 0 0 0 B 0 0 0 0 0 O 0 0 N o 0 3 0 1 1 2 0 0 No s N 0 O 0 0 0 0 O 0 B 0 0 0 0 0 0 0 0 N o 0 2 0 0 1 0 0 0 No S N o 0 1 0 0 0 0 0 0 B 0 4 0 0 0 0 0 0 exhibit high adhesive characteristics, this property is of short duration and of little commercial significance. Apparently, significant amounts of oxides or other compounds are formed on the surfaces of the cleaned metal sheets during the prebaking operation so as to render the cleaning treatment less effective. Cleaned but unsteamed magnesium-aluminum articles, at some time after being subjected to the atmospheric conditions prevalent in commercial forming plants, also exhibit the drastically poor results obtained by the testing of the noncleaned alloy materials as shown in Table I. It is believed that the presence of magnesium carbonate on the surface of magnesium aluminum alloys is a main interfering constituent which causes the lack of adhesion between the surface of the untreated alloy and organic enamel coatings.

A significant factor appearing from the results of Tables I and II is that the steaming of a cleaned surface of a magnesium-aluminum alloy produces a chemical action or in some way renders the surface of the alloy extremely adherent to organic enamel coatings even though such treated alloys were prebaked after steam treatment. A stable film is thus produced on the surface of the magnesium-aluminum alloy in an extremely short period of time. The same time definitely was not long enough for the development of a useful polymeric hydrated oxide film on the comparable surface of the aluminum samples which contained no magnesium.

Thus, by utilizing the processes of the present invention, a film which satisfactorily increases the permanent ability of organic enamels to adhere to a magnesiumaluminum alloy surface is generated in an extremely short period of time. There is strong evidence pointing toward the formation of a polymeric hydrated oxide film on the surface of the magnesium-aluminum alloy which contains both magnesium and aluminum within the basic polymer unit. Regardless, however, of the nature of the film formed, the instant process, accomplished in a few seconds with magnesium-aluminum alloys, results which pre viously took from 1 to 20 hours of treatment with nonmagnesium aluminum alloys.

The commercial advantage is that magnesium aluminum alloys are thus made useful for containers for food products.

Specific illustrative examples of the invention carried out upon commercial sheet stock are as follows:

Example 1 (a) Immerse 30-45 seconds in an alkali detergent bath at 82 C. (100 ounces of sodium hydroxide in 100 gallons of tap water);

(b) Rinse 15 seconds by spray of cold tap water;

(c) Immerse 3045 seconds in 20% nitric acid solution (3.6 volumes of 69-71% HNO to 14.4 volumes of deionized water);

(d) Rinse 15 seconds by spray of cold tap water;

(e) Rinse 3 seconds by spray of cold deionized water;

(f) Squee-gee to remove water film;

(g) Expose for seconds to saturated steam at 100 C.;

(h) Squee-gee to remove condensate;

(i) Force-dry by gas flame.

Example 2 (a) and (b) as in Example 1;

(c) Immerse 3045 seconds in aqueous sulfuric acid solution (1 Normal) at room temperature;

(d) through (i) as in Example 1.

Example 3 (a) through (f) as in Example 1;

(g) Immerse in boiling deionized water for 5 seconds; (h) Squee-gee to remove water film;

(i) Force-dry by gas flame.

Example 4 (a) through (f) as in Example 2; (g) through (i) as in Example-3;

Example 5 1 Example 6 (a) Abrade metal surface to clean as in Example 5;

(b) Expose for 5 seconds to saturated steam at 99- 100 C.;

(c) Squeegee to remove condensate.

(d) Force dry by gas flame.

The sheets of magnesium-aluminum alloy after being treated as set forth in the above illustrative Examples 1-6 were coated with standard organic enamels. The thus coated sheets were then baked according to the normal schedule for each particular enamel and subsequently 5 formed into can bodies. The adherence of each organic enamel to each magnesium-aluminum alloy substitute was found to be excellent for all commercial aspects.

It is obvious that the specific examples are not restrictive, and that the invention may be practiced in other ways within the scope of the appended claims.

What is claimed is:

1. A method of improving the adhesion between magnesium-aluminum alloys wherein the aluminum is present in a prevailing amount and an organic enamel coating which comprises (1) cleaning the surfaces of said magnesium-aluminum alloys to remove existing oxides and (2) subsequently subjecting the cleaned surfaces to water having a temperature of at least C. for a period of about 2 to 30 seconds to form a stable oxide film of magnesium and aluminum which is receptive to said organic enamel coating.

2. The method of claim 1 further characterized in that the magnesium-aluminum alloy surfaces are cleaned by mechanical abrasion.

3. The method of claim 1 further characterized in that the water is in the form of saturated steam.

4. The method of claim 1 further characterized in that the magnesium-aluminum alloy surfaces are cleaned by immersing the surfaces of the alloy in an alkali bath followed with water rinses and an immersion in an acid bath.

5. The method of claim 1 further characterized in that the magnesium-aluminum alloy contains approximately 2.5 to 4% by weight of magnesium.

6. The method of claim 1 further characterized in that the cleaned magnesium-aluminum alloy surfaces are subjected to water having a temperature of at least 100 C. for a period of about 2 to 5 seconds.

'7. A method of preparing an adherent organic enamel coating on surfaces of magnesium-aluminum alloys wherein the aluminum is present in a prevailing amount which comprises pretreating said surfaces before applying the organic coating; said pretreatment comprising (1) cleaning the magnesium-aluminum alloy surfaces to remove existing oxides and (2) subsequently subjecting the cleaned surfaces to Water having a temperature of at least 100 C. for a period of about 2 to 30 seconds to form an oxide film of magnesium and aluminum which is receptive to said enamel coating.

References Cited by the Examiner UNITED STATES PATENTS 1,451,755 4/1923 Backer 1486.27 2,373,937 4/1945 Allen et a1 1486.3 2,531,382 11/1950 Arditi 1486.3 2,620,287 12/1952 Bramley 148-63 2,762,694 9/1956 Newman 117-49 2,859,148 11/1958 Altenpohl 1486.27 3,032,435 5/1962 Michel 148-6.27 X 3,039,898 6/1962 Keller 148-6.27

JOSEPH B. SPENCER, Primary Examiner.

RICHARD D. NEVIUS, Examiner. 

1. A METHOD OF IMPROVING THE ADHESION BETWEEN MAGNESIUM-ALUMINUM ALLOYS WHEREIN THE ALUMINUM IS PRESENT IN A PREVAILING AMOUNT AND AN ORGANIC ENAMEL COATING WHICH COMPRISES (1) CLEANING THE SURFACES OF SAID MAGNESIUM-ALUMINUM ALLOYS TO REMOVE EXISTING OXIDES AND (2) SUBSEQUENTLY SUBJECTING THE CLEANED SURFACES TO WATER HAVING A TEMPERTURE OF AT LEAST 100*C. FOR A PERIOD OF ABOUT 2 TO 30 SECONDS TO FORM A STABLE OXIDE FILM OF MAGNESIUM AND ALUMINUM WHICH IS RECEPTIVE TO SAID ORGANIC ENAMEL COATING. 